The present invention relates to the synthesis of methanol, a fast and strongly exothermic catalytic reaction, according to the reactions: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH
and/or EQU CO.sub.2 +3H.sub.2 .revreaction.CH.sub.3 OH+H.sub.2 O.
These reactions are so strongly exothermic that in most cases they require the use of a diluent, associated or not with a thermal exchange through the wall of the reactor. Several processes have been designed which make use, as diluent, either of the output gaseous mixture which is then partially recycled, or of a liquid, for example a hydrocarbon, which, under the operating conditions, can remove the heat by direct heat exchange and evaporation.
In the processes operated with a liquid, the catalyst is normally used as a suspension of fine particles, as a fixed bed or as an ebullated bed.
It is important that the catalyst be immersed in the liquid to ensure a good liquid-solid contact everywhere, thereby avoiding the formation of a dry zone in the reactor, which zone could be responsible for an insufficient thermal stability; very high temperatures can be attained in that dry zone, since the reaction can take place entirely in the gas phase therein. A number of techniques have been proposed, wherein a continuous liquid phase is circulated upwardly, the catalyst being used as a suspension, as an ebullated bed or as a fixed or moving bed. These techniques have however serious disadvantages: substantial attrition of the catalyst takes place, due to the displacement of the catalyst particles, even with a so-called fixed bed; fine catalyst particles can be carried away from the reaction zone, resulting in racing of the reaction in the separators, the exchangers or at any other part of the plant.
Although the synthesis of methanol has been disclosed in the prior art (see, for example, U.S. Pat. No. 2,535,060 and U.S. Pat. No. 4,031,123), it has not been appreciated that fixed bed operations could be improved by operating at high velocities of the gas phase and of the liquid phase.
Thermal instability can be sometimes tolerated by the catalyst for a certain time, when it is weak. However it unavoidably results in a shortening of the catalyst life, and a decrease in both activity and selectivity to methanol.